In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source. Specifically, ultraviolet radiation such as g-line and i-line radiation has conventionally been used, but nowadays KrF excimer lasers and ArF excimer lasers have begun to be introduced in mass production of semiconductor elements. Also, radiations having a shorter wavelength than that of these excimer lasers, for example, F2 excimer laser, electron beams, EUV (extreme ultraviolet), and X-ray are also being examined.
One example of a pattern forming materials that can form a pattern of minute dimensions is a chemically amplified resist composition including a base material having film formability and an acid generator which generates an acid upon exposure. Chemically amplified resists include negative resists in which alkali solubility decreases upon exposure, and positive resists in which alkali solubility increases upon exposure.
Polymers have conventionally been used as a base material component of these chemically amplified photoresists. For example, there have been used polyhydroxystyrene (PHS) or a PHS-based resin in which a portion of the hydroxyl groups are substituted with acid dissociable, dissolution inhibiting groups, and a copolymer derived from a (meth)acrylate ester or a resin in which a portion of the carboxyl groups are substituted with acid dissociable, dissolution inhibiting groups.
However, when a pattern is formed using such a pattern forming material, there is a problem that roughness occurs on the surface of a top face and a side wall of the pattern. For example, roughness on the surface of the side wall of the resist pattern, namely, line edge roughness (LER) is causative of distortion around holes in a hole pattern and unevenness of a line width in a line-and-space pattern and thus it may exert an adverse influence on formation of minute semiconductor devices
Such a problem becomes more severe as the pattern size becomes smaller. Therefore, since lithography using electron beams and EUV aims at forming a pattern of minute dimensions of several tens of nanometers, ultralow roughness more than the present pattern roughness is required.
However, polymers used as the base material usually have a large molecular size (mean square radius per molecule) such as about several tens of nanometers. In a developing step of pattern formation, development of a resist to be dissolved in a developing solution is usually performed in one molecular unit of a base material component. Therefore, so long as a polymer is used as a base material component, it is very difficult to further reduce the roughness.
To cope with such problems, there is proposed, as the material aiming at ultralow roughness, a resist in which a low molecular material is used as the base material component. For example, patent documents 1 and 2 propose a low molecular material containing alkali-soluble groups, such as hydroxyl groups, in which either a portion of, or all of, the groups are protected with acid dissociable, dissolution inhibiting groups. It is expected that such a low molecular material can reduce the roughness because of the small molecular size attributed to a low molecular weight.
(Patent Document 1)
Japanese Unexamined Patent Application, First Publication No. 2002-099088
(Patent Document 2)
Japanese Unexamined Patent Application, First Publication No. 2002-099089